FIG. 1 shows a prior art disk drive comprising a disk 2, a head 4 actuated over the disk 2 by a voice coil motor (VCM) 6, and a multi-phase spindle motor 8 for rotating the disk 2. The multi-phase spindle motor 8 comprises a plurality of windings (e.g., φA, φB, φC) which when driven with current generate a magnetic field that interacts with the magnetic field of permanent magnets (not shown) to generate a rotational torque. The disk 2, head 4, VCM 6, and spindle motor 8 are typically housed in a head disk assembly (HDA) 10, wherein control circuitry 12 mounted on a printed circuit board is typically attached to the HDA 10.
The control circuitry 12 commutates the windings of the spindle motor 8 over commutation intervals using any suitable commutation sequence. For example, commutation logic 14 may control switches 16 to commutate the windings of the spindle motor 8 in a two-phase, three-phase, or hybrid two-phase/three-phase commutation sequence. A commutation controller 18 applies a control signal 20 to the commutation logic 14 in order to transition between the commutation states.
The windings of the spindle motor 8 are connected to a back electromotive force (BEMF) detector 22 which detects threshold crossings (e.g., zero crossings) in the BEMF voltage generated by the windings with respect to the center tap. Since the BEMF voltage is distorted when current is flowing, the commutation controller 18 signals the BEMF detector 22 over line 23 when an “open” winding is generating a valid BEMF signal. At each BEMF threshold crossing the BEMF detector 22 toggles a signal to generate a square wave signal 24. The frequency of the BEMF threshold crossings and thus the frequency of the square wave signal 24 represent the speed of the spindle motor 8. The commutation controller 18 evaluates the square wave signal 24 and adjusts the control signal 20 in order to control the speed of the spindle motor 8.
The spindle motor 8 may be driven using any suitable driving signals, such as with square or trapezoidal waves, but the preferred driving signal is a sinusoidal wave. Example sinusoidal driving signals applied to the windings of the spindle motor 8 are shown in FIG. 2, wherein the frequency of the sinusoids is controlled closed loop in response to the BEMF signal 24. Each sinusoidal signal may be applied to a spindle motor winding using a linear amplifier, or each sinusoidal signal may modulate a PWM signal for driving the respective switches 16 shown in FIG. 1.
When the disk 2 is spun up after the disk drive is powered on or exits an idle mode, the BEMF signal 24 is not a reliable velocity sensor. Therefore, the prior art typically employs some other form of closed loop feedback to spin-up the disk 2, such a technique referred to as inductive sense. With inductive sense, the BEMF detector 22 periodically senses the angular position of the spindle motor 8 and generates a corresponding inductive sense signal 26. The commutation controller 18 processes the inductive sense signal 26 in order to commutate the windings of the spindle motor 8 at the appropriate time. A problem with using inductive sense as closed loop feedback during spin-up is the acoustic noise generated each time the driving signals are disabled (or modified) in order to make an inductive sense measurement.